There where 2 main problems I found using openweathermap for mine. First you need to include an api key in the URL otherwise calling from a different domain is rejected, you can get that by signing up for free on their site. The other problem was that it wouldn’t work if I was making the call from a HTTPS site. I’m not sure how to solve this problem though so if any one has I would appreciate being pointed in the right direction. For now I’m just detecting that the site is loaded over HTTPS and displaying a message to say it wont work with it.
This thread is also related to the problems: http://forum.freecodecamp.com/t/im-confused-with-all-the-json-jsonp-localhost-and-cors-concepts/11365/2